Once a system has been started by some external power source, the working fluids sustain the process on a self sustaining basis, such a system is known as a bootstrap system. A gas turbine engine once started, sustains a bootstrap operation, because the turbine continues to drive the compressor that feeds air to the engine. Another bootstrap system used by aircrafts is the bootstrap air conditioning system which provides adequate cooling to the cabin and electronic equipment.
The bootstrap air conditioning system mainly consists of two units, the cold air unit and a heat exchanger, both of which are mounted on a common shaft. This assembly was supported on ball bearings until they were replaced by air bearings which provide a lighter solution, requiring less maintenance. The compressor increases the air pressure with a corresponding increase in temperature. The increased temperature is now reduced in the heat exchanger which is cooled by ram air. This reduction in temperature might lead to formation of water, especially when the aircraft is flying in humid conditions. The water extractor placed at the turbine inlet removes most of the water thus preventing damage to the turbine blades and spraying water into the cabin and electronic equipment. As the air expands across the turbine, the temperature can drop below 0o and hence a cold air bypass line is used to vary turbine outlet temperature to get the required temperature for cabin and equipment cooling.
